[Intel-gfx] [PATCH] [RFC] drm/i915: read-read semaphore optimization

[Eric Anholt]

On Mon, 12 Dec 2011 19:52:08 -0800, Ben Widawsky <ben at bwidawsk.net> wrote:
> Since we don't differentiate on the different GPU read domains, it
> should be safe to allow back to back reads to occur without issuing a
> wait (or flush in the non-semaphore case).
> 
> This has the unfortunate side effect that we need to keep track of all
> the outstanding buffer reads so that we can synchronize on a write, to
> another ring (since we don't know which read finishes first). In other
> words, the code is quite simple for two rings, but gets more tricky for
> > 2 rings.
> 
> Here is a picture of the solution to the above problem
> 
> Ring 0            Ring 1             Ring 2
> batch 0           batch 1            batch 2
>   read buffer A     read buffer A      wait batch 0
>                                        wait batch 1
>                                        write buffer A
> 
> This code is really untested. I'm hoping for some feedback if this is
> worth cleaning up, and testing more thoroughly.

You say it's an optimization -- do you have performance numbers?
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